This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
Weiliu, you're getting unexpected result because hash set does not guarantee the order of its elements because it's based on hashing. If you want to enforce the results, you can use TreeSet. Best regards, -Asif